Everybody itches. Sometimes itch serves as a useful warning signal — there's a bug on your back! But sometimes itch arises for no apparent reason, and can be a torment.

Think of the itchy skin disorder eczema, or the constant itching caused by some cancers. "A very high percentage of people who're on dialysis for chronic kidney disease develop severe itch that's very difficult to manage," says Dr. Ethan Lerner, an associate professor of dermatology at Harvard Medical School.

Scientists now say they've got a much better clue as to how itch happens.

For a long time, the thought was that itch piggybacked on the nerves that feel pain or temperature. But it now looks like itch has its own dedicated highway from skin to brain.

And the molecule that makes itch happen comes as a surprise; it usually hangs out in the heart, where it helps control blood pressure.

It's a neurotransmitter called natriuretic polypeptide B, or Nppb.

Researchers at the National Institute of Dental and Craniofacial Research wondered what it was doing in nerve cells. To find out, they created a mouse that didn't make Nppb in its body.

Things that made a normal mouse scratch like crazy had no effect on mice with no Nppb. But when those mice were injected with the substance, they scratched, too.

Nppb seems to be working sort of like an itch-molecule. Take it away and the mice don't itch. Put it back and the itch returns.

The researchers also found a small group of nerves in skin that produce and use this molecule to send an itch message to the spinal cord.

This research hasn't been replicated in humans, so it doesn't prove that human itch works the same way. But the researchers are confident that the molecule is a key clue in defining the long-elusive itch pathway.